In a modern, high-power turbojet with a high by-pass ratio, the fan has a large diameter, and the peripheral speed at the tips of the blades is proportional both to this diameter and to the speed of rotation of the fan. In order to obtain good efficiency, the peripheral speed must be lower than the speed of sound. In a conventional turbojet having a single fan, this is made possible by interposing reduction gearing between the drive shaft and the fan. Nevertheless, gearing increases the weight of the engine considerably and also reduces efficiency. Another technique consists in equipping the turbojet with two fans, a front fan and a rear fan, said fans being assembled at the front of the intermediate casing, and each fan being driven by a respective low-speed drive shaft, without any interposed reduction gearing. Each fan supplies the air flowing in the by-pass channel with substantially half the propulsion power of the by-pass air.
The state of the art is illustrated, in particular, by U.S. Pat. Nos. 3,861,139 and 4,860,537, which describe turbojets of the type referred to in the introduction, each having two counter-rotatable fans, which fans are connected to a low-pressure compressor also having counter-rotatable rotors, one of the rotors being driven by the drive shaft for the front fan and the other rotor being driven by the drive shaft for the rear fan. The fixed pitch of the blades of the two fans is determined to optimize matching of the two fans to a particular engine speed. At other speeds, however, efficiency is necessarily reduced.